The macbook drives my external screens (27" and a portrait orientated 24") as well as the retina and never skips a beat. It has the top of the range i7 processor, 16GB of RAM and a fast SSD.

External data is stored in USB 3.0 enclosures which are way faster than the mechanical drives inside them. There's no need to go thunderbolt unless you're going with uber fast RAID or external SSDs.

You probably don't need the dual GPU beasts in the MP, so buying a MacPro will 'initially' not give you the same 'bang for your buck' as (even a new once they update it) Mac Mini. But software a few months down the line may change that for you. Personally I'd love a MacPro but I'd rather rip out one of the GPUs if I could have more CPU grunt. For what I do NOW the CPU is more important than the GPU. I'm not editing 4K.

If you buy a Windows PC, you'll regret it. Those of us that use dual systems tend to prefer OSX and the hassle free experience to any flavour of Windows. I know it. You know it.

My advice?

Either wait for a new Mac Mini to be released and see if that works for you, or buy a second MacBook (Air?) to use when the retina is docked to your big screen(s). You could even buy a second retina for less than a MP.

Remember that MacBooks tend to hold their value really well, so even if you change your mind the money you'd lose by selling on is nothing compared to what you'd lose by buying and selling a PC.

PS I keep the PC around purely for gaming duties. It doesn't have as much RAM as the Mac, it's SSD is smaller and slower and it's i7 isn't quite as fast... but it's GPU is amazing and means I can shoot zombies and aliens... or zombie aliens in 1440p. Guilty as charged.